So, i have power going to the VCO now, but it has a few problems that I can not figure out for the life of me :/

i'm still trying to get the modulator working atleast.

The Modulator:This part seems to only want to work sometimes when i turn it on, and it only cycles when the attack is at a certain point. (Anything above the 8 o' clock position, the led freezes and the modulation stops.) The release effects it, but when turned all the way up, it does NOT take very long to dim back down. And with the output all the way up, i get a few semitones of change (if that) when the mod out isn't patched to anything. when it's patched to P1 of either oscillator or PW it gets a good amount of modulation, though. LED seems to turn on randomly every now and then when playing with the oscillators

Oscillator 1:This seems to be tuned very low, oscillator also seems to jump pitches quite often, it's very unstable. LFO seems to work when the knob is pass the 3 o clock position. All waves work like this and are the same pitch. Sine wave is also distorted

Oscillator 2:This one seems pretty stable for the most part, it doesn't jump pitches and if it does its not much. Pitch knob gives it a few semitones of pitch, not octaves though. Pulse width works great. All waves work and are at the same pitch.

Overall:I'm getting a lot of bleed from the oscillators.

I've checked all of my solder joints and they seem fine, everything appears to be grounded correctly, and everything appears to be in the correct place. I'll go over it once again to make sure its facing the right way/in the correct place. I have yet to check the resistors, I'll do that too.

Any suggestions and help would be GREATLY appreciated.

Tue Feb 08, 2011 1:55 pm

PAiA-Scott

Joined: Sun Nov 09, 2008 2:49 pmPosts: 732

Re: 9720 VCO trouble shooting

Check for the power supply voltages from the two three-pin voltage regulator ICs.

Maybe one of the 14pinICs is in place of another??? The TL 084 op-amps can fail if there is an accidental short-circuit between pins (the bottom of the board touching the chassis metal or a tool while powered).

This is very frustrating. I really hope i can figure out what's going on. I feel like i'm spending hours staring at this thing and finding nothing.

Thu Feb 10, 2011 12:30 am

PAiA-Scott

Joined: Sun Nov 09, 2008 2:49 pmPosts: 732

Re: 9720 VCO trouble shooting

Thanks for posting the video. It seems like it's close, but there shouldn't be the interaction between the A and B sections when the Pitch A is adjusted, and, the AR Release should have more affect on the time and the Attack shouldn't cause the cycle to stop.

The variations in voltages you list don't really tell so much and could be attributable to the condition that's causing the interaction between sections and AR operation not being completely right.

Looking at the assembly manual illustration supplement pages, figs 1 and 2, notice that the AR modulator circuitry is at the panel end of the board and the panel connections for the VCO outputs are very near where the board meets the panel. Maybe a wire or terminal or a wire extending through a terminal is contacting one of the parts on the board.

Another way things can interact when they shouldn't or cause a section to malfunction or be compromised is if a panel point-to-point wiring path soldering connection isn't soldered, or, the solder did not flow to all wires or component leads in the joint (or a terminal/lug). Nudge the bare wire links on the panel wiring watching for any movement in the joints--a sign that the joint needs more heat/solder.

Figs. 3, 4, and 5 are useful for confirming the panel connections. Look to be sure the pots for Rs 203, 204, and 206 have the same markings, and, Rs 200 and 201.

Inspect closed-circuit connectors shown with the shunt terminal (X) for a possible bulge or deformation in the black plastic body--it melting from heat during soldering can cause the shunt to not make it's normally closed contact with the Tip (T). J10 is a connector like this for normally connecting the modulator output to the VCO B Pulse Width control voltage input. J11 works to link the VCOA Pitch control to the VCOB pitch when there is not a plug inserted to substitute a CV for the VCOA P1 control input. J3 is another, used for the VCO A PW cv input.

Sometimes if I can see photos clearly showing the panel connections and top and bottom sides of the board, I can reply about something that looks out of order or possibly in need of attention.

Let me know if you'd just prefer to get the trouble corrected and I'll send the pdf service authorization/information form for sending it here.

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